Dumping Very Large Java Heaps

When a java application has either a memory leak or much higher than expected memory utilization, it is necessary to obtain heap information to identify the source of the problem.  A heap dump is ideal because it can then be analyzed using various tools.  However, with very large java heaps, perhaps > 100GB, a heap dump may be impractical for several reasons:

• the heap dump may crash the java process before completing
• the heap dump may hang indefinitely
• there may not be enough disk space to accomodate the dump
• the dump may be so large that analysis tools are unable to process it

One solution to this scenario is to use the jmap utility to obtain a heap dump histogram from the running process.  This appears to be very lightweight, completing quickly on very large heaps and generating a very small summary analysis file that can be used for troubleshooting.

The syntax for doing this is the following, where <pid> is the process id of the java process.

jmap -histo <pid>

The output is a very nice summary showing, for each class in the heap, the class name, the number of instances, and the size in bytes, for example as follows:

 num     #instances         #bytes  class name

----------------------------------------------

   1:         70052       11118624  <constMethodKlass>

   2:         70052        8422160  <methodKlass>

   3:          6320        8258472  <constantPoolKlass>

   4:          6320        6117216  <instanceKlassKlass>

   5:        116656        5732520  <symbolKlass>

   6:         17467        5729824  [I

   7:          5682        5050352  <constantPoolCacheKlass>

   8:         57275        4818512  [C

   9:         24818        2660384  [B

  10:         59327        1898464  java.lang.String

  11:          2847        1766720  [J

  12:          2978        1542008  <methodDataKlass>

  13:         11687         797256  [S

  14:         13307         706440  [Ljava.lang.Object;

  15:          6777         704808  java.lang.Class

  16:         18904         604928  java.util.HashMap$Entry

  17:         10088         522512  [[I

  18:          5736         499408  [Ljava.util.HashMap$Entry;

  19:         12838         410816  java.util.Hashtable$Entry

  20:          5580         267840  java.util.HashMap

  21:           428         249952  <objArrayKlassKlass>

  22:          5888         235520  java.util.concurrent.ConcurrentHashMap$Segment

  23:          6243         199776  java.util.concurrent.locks.ReentrantLock$NonfairSync

  24:          5888         146544  [Ljava.util.concurrent.ConcurrentHashMap$HashEntry;

  ...

3029:             1             16  sun.awt.X11.XToolkit$4

3030:             1             16  java.util.Collections$EmptyIterator

3031:             1             16  com.sun.tools.visualvm.core.explorer.ExplorerContextMenuFactory

3032:             1             16  sun.reflect.generics.tree.TypeVariableSignature

3033:             1             16  sun.awt.X11.XKeyboardFocusManagerPeer$1

3034:             1             16  org.openide.xml.EntityCatalog$Forwarder

Total        714547       74730032